Pothead



June 1o, 1930.

P. F. WILLIAMS POTHEAD Filed Feb. 19, 1925 @WwW/@g3 Patented `lune l0, 11930 UNITED sTATEs PATENT oEElcE PAUL F, WILLIAMS, 0F CHICAGO, ILLINOIS, ASSIGNOR TO G & W ELECTRIC SPECIALTY COMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS romanian Application tiled February 19, 1925. Serial No. 10,210.

This invention relates to potheads.

One object of the invention is to provide a simple, safe, durable and eiicient pot-head arrangement whereby the portion of c able within the pothead not covered by the sheath is as safe against rupture as the portion covered by the sheath.

Another object is to sheathe a portion of a cable in a pothead and terminate the sheath therein in an improved manner to reduce the tendency to rupture and to offer maximum resistance to rupture.

Another object is to provide a pothead arrangement adapted to meet all of the requirements for successful commercial use.

These and other objects are accomplished by means of the arrangement disclosed on the' accompanying sheet of drawings, in which Figure 1 is a vertical longitudinal sectional view through a pothead embodying my invention;

Figure 2 is an'elevational view of the same pothead taken at right angles to the pothead as shown in Figure l, and ,x Figure 3 is a diagrammatic view showing the direction which the dielectric linesY of force are caused to take when the end of the sheath is tapered.

The various novel features of the invention will be apparent from the following description and drawings, and will be particularly pointed out in the appended claims.

Referring to the figures of the drawings, it will be noted that I have illustrated my invention in connection with a pothead including a metal cup 10 forming part of a bracket 11, by means of which the pothead may be attached to any suitable supporting structure. Mounted upon the projecting reduced rim portion 12 of the cup is an insulating tube 13 having the usual petticoats 14. The lower end of the tube terminates in a cylindrical portion 15 and extends downwardly within the cup 10. A suitable gasket 16 is placed between the cup portion 12 and the cooperating shoulder portion 17 o f the tube. The lower end vof the cup 10 1s tapered downwardly and has connected thereto a wiping sleeve 18, there being a gasket 19 interposed between said part-s. A cable 20, including a conductor 21 and insulation 22 wrapped thereabout, extends upwardly through the sleeve 18, cup 10, tube 13 and is electrically and mechanically connected to a cap 23, which fits over the top of the tube 13. A suitable gasket 24 is interposed between the top petticoat of the tube 13 and the cap 23, there being a layer of cement 25 for sealing the cap on said tube. The conductor 21 is electrically connected to a hooded lug 26 mounted over the cap 23 and a cable lug 27 in turn is releasably connected to the hooded lug at 28.`

A metal sheath, such for example as a lead sheath 29, surrounds or covers that portion of the cable 20 which extends from the ground and passes through the sleeve 18 and cup 110 and terminates above said cup 1() within the tube 13. A wipe 30 forms a seal between the sheath 29 and the lower tapered portion of the sleeve 18. Liquid dielectric or insulating material is forced into the pothead and fills all space therein, 'said liquid dielectric material preferably being forced into the pothead through a tube 31 connected to the cup 10, the displaced air being ejected through openings 32 in the cap 23, which openings are normally closed by plugs 33. It will be noted that the cap 23 is provided with considerable space for the storage of dielectric material and, if necessary, an additional amount may be supplied thereto, and also to the tube 13 and cup 10 through connection 31.

This pothead is designed particularly for use in connection with high potential current carrying cables. There is a well-established ruleI that an-electric conductor of a given diameter shall be wrapped with a given thickness of a given insulating material to obtain the greatest resistance to breakdown of the insulation. If less insulation is used, the resistance to breakdown usually is decreased and likewise, if an increased amount of. insulation is used, especially where the dielectric constant is different, the resistance to breakdown is reduced. An insulated cable passing through a pothead is surrounded by a dielectric insulating material to hermetically seal the cable Within the pothead. As mentioned before, the pothead cup 10 is of metal, and in addition to the insulation around the cable 21 an insulating liquid is forced into the cup and between its walls and the tube and the cable. In so far as the liquid insulating material within the cup is concerned, it may be considered excessive in amount, even to the point of making the total dielectric strength between the cable conductor 21 and the cup 10 weaker than it would be if only the insulating material 22 around the conductor 21 were used. Accordingly the metal sheath 29 is extended completely through the cup 10 and terminates at a point upwardly beyond the rim of said cup. Both the metal cup 10 and the sheath 29 are grounded, and accordingly there is no difference in potential between said cup and sheath. In this way the pothead and/or the sheathing is made more nearly punctureproof.

In order to further safeguard the pothead against puncture, the upper end or member 35 of the sheath is tapered away from the cable, thereby causing the dielectric lines of force at the upper termination of the sheath to be spread out and travel in lines at right angles to the insulation 22, which is wrapped around the cable conductor 21. It is a known fact that the dielectric strength or resistance-to breakdown crosswise of wrapped insulation is ten times greater than the dielectric strength lengthwise of said insulation. Ac-

cordingly, by tapering the end of the sheathaway from the conductor, the lines of force in question are spread out and greater resistance to the dielectric lines of force is presented. In Figure 3 this point is illustrated somewhat diagrammatically, wherein it is noted that the dielectric lines of force 36 are shown astraveling in spread out lines more at right angles with respect to the tapered end of the sheath than dielectric lines of force would travel toy ward an untapered end of the sheath. The

advantage of providing the cable sheathing with the member 35 may be further appreciated when comparing concentration of lines of force at spaced points and the concentration of lines of :torce due to the same voltage between rounded members. It is a known fact that lines of force will be concentrated at points or sharp edges of members rather than on rounded edges of members when the spacing of said members is equal, and further it is possible to space points at distances from sixteen to twenty times greater than distances between rounded edges of members to cause the same stresses to be concentrated between members and the points for the same voltage. In other words, if the member 35 were not rounded, lines of force would be set up from distances along the cable so far remote from the member 35 that the lines of force will run approximately parallel to the layers of insulation, tending to break down this insulation, whereas by rounding the edge Vof the member 35 as illustrated in Figures 1 al, such as lead, used in the main body of the sheath 29 might betapered away, but it is not as mechanically strong as brass or other conductors which may be used in the tapered away portion 35.

By means of the arrangement hereinabove described a pothead is provided which is adapted to meet all of the requirements for succes ul commercial use.

I claim:

1. 1n a device of the class described, the combination of a pothead attaching bracket having a cup, an insulating tube mounted upon and sealed with respect to said cup and being of a smaller internal diameterthan said cup, an insulated cable conductor extending through said cup and tube, and a metal sheath covering said cable and entering the bottom of said cup, passing therethrough and extending to a point below the upper edge of said cup and within said tube to decrease the possibility of rupture Iot the insulation of said cable, said end ot the sheath being provided with conducting means supported by the sheath and tapered away from the cable to present the'greatest resistance to the dielectric lines of force which tend to rupture the insulation.

2. In a device of the class described, the combination of a pothead attaching bracket having a cup, an insulating tube mounted upon and sealed with respect to said cup and being of a smaller internal diameter than said cup, an insulated cable conductor extending through said cup and tube, a metal sheath covering said cable and entering the bottom of said cup, passing therethrough and extending to a point below the upper edge of said cup and within said tube to decrease the possibility of rupture of the insulation of'isaid cable and pothead, said end of the sheath being provided with conducting means supported by the sheath and tapered away from the cable to present the greatest resistance to the lines of force which tend to rupture the insulation, a metal cap for said tube electrically -connected to said conductor, and insulating material within said tube between said sheath and cap and the sheath and cup to prevent y an appreciable distance into the insulating bushing and terminating therein, a flared member secured to said end of the sheathing and spaced from said bushing, the termina'- tion of the flare of said last named member beingia rounded portion to prevent concentration of lines ot' force tending to disrupt the conduct-or insulation.

4. In a pothead, the combination of a metallic supporting cup, a hollow insulating bushing supported by and having a portion extending an appreciable distance into said cup, a metallic cap for said bushing, a metallic sheathed insulated conduct-or extending into said cup and bushing and being electric-illy connected to said ap, the metallic sheathing of said conductor extending a material Adistance into said bushing and terminating therein, a metallic member secured to the termination of said sheathing to form a continuation thereof, said metallic member tapering away from said conductor and terminating in a rounded bead at a distance 1naterially above said cup whereby concentration of lines of force tending to disrupt the conductor insulation is prevented, said cup being' secured to said sheathing and being free to move with said sheathing and conductor whereby relative movements between the parts of the sheathed conductor tending to disrupt said parts is prevented.

Signed at Chicago, Illinois, this 13th day of February, 1925.

PAUL l". VILLIAMS. 

